pack-bitmap-write: use existing bitmaps

When constructing new bitmaps, we perform a commit and tree walk in
fill_bitmap_commit() and fill_bitmap_tree(). This walk would benefit
from using existing bitmaps when available. We must track the existing
bitmaps and translate them into the new object order, but this is
generally faster than parsing trees.

In fill_bitmap_commit(), we must reorder thing somewhat. The priority
queue walks commits from newest-to-oldest, which means we correctly stop
walking when reaching a commit with a bitmap. However, if we walk trees
interleaved with the commits, then we might be parsing trees that are
actually part of a re-used bitmap. To avoid over-walking trees, add them
to a LIFO queue and walk them after exploring commits completely.

On git.git, this reduces a second immediate bitmap computation from 2.0s
to 1.0s. On linux.git, we go from 32s to 22s. On chromium's fork
network, we go from 227s to 198s.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap-write.c

@@ -340,20 +340,37 @@ static void fill_bitmap_tree(struct bitmap *bitmap,
 
 static void fill_bitmap_commit(struct bb_commit *ent,
 			       struct commit *commit,
-			       struct prio_queue *queue)
+			       struct prio_queue *queue,
+			       struct prio_queue *tree_queue,
+			       struct bitmap_index *old_bitmap,
+			       const uint32_t *mapping)
 {
 	if (!ent->bitmap)
 		ent->bitmap = bitmap_new();
 
-	bitmap_set(ent->bitmap, find_object_pos(&commit->object.oid));
 	prio_queue_put(queue, commit);
 
 	while (queue->nr) {
 		struct commit_list *p;
 		struct commit *c = prio_queue_get(queue);
 
+		if (old_bitmap && mapping) {
+			struct ewah_bitmap *old = bitmap_for_commit(old_bitmap, c);
+			/*
+			 * If this commit has an old bitmap, then translate that
+			 * bitmap and add its bits to this one. No need to walk
+			 * parents or the tree for this commit.
+			 */
+			if (old && !rebuild_bitmap(mapping, old, ent->bitmap))
+				continue;
+		}
+
+		/*
+		 * Mark ourselves and queue our tree. The commit
+		 * walk ensures we cover all parents.
+		 */
 		bitmap_set(ent->bitmap, find_object_pos(&c->object.oid));
-		fill_bitmap_tree(ent->bitmap, get_commit_tree(c));
+		prio_queue_put(tree_queue, get_commit_tree(c));
 
 		for (p = c->parents; p; p = p->next) {
 			int pos = find_object_pos(&p->item->object.oid);
@@ -363,6 +380,9 @@ static void fill_bitmap_commit(struct bb_commit *ent,
 			}
 		}
 	}
+
+	while (tree_queue->nr)
+		fill_bitmap_tree(ent->bitmap, prio_queue_get(tree_queue));
 }
 
 static void store_selected(struct bb_commit *ent, struct commit *commit)
@@ -386,6 +406,9 @@ void bitmap_writer_build(struct packing_data *to_pack)
 	size_t i;
 	int nr_stored = 0; /* for progress */
 	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
+	struct prio_queue tree_queue = { NULL };
+	struct bitmap_index *old_bitmap;
+	uint32_t *mapping;
 
 	writer.bitmaps = kh_init_oid_map();
 	writer.to_pack = to_pack;
@@ -395,6 +418,12 @@ void bitmap_writer_build(struct packing_data *to_pack)
 	trace2_region_enter("pack-bitmap-write", "building_bitmaps_total",
 			    the_repository);
 
+	old_bitmap = prepare_bitmap_git(to_pack->repo);
+	if (old_bitmap)
+		mapping = create_bitmap_mapping(old_bitmap, to_pack);
+	else
+		mapping = NULL;
+
 	bitmap_builder_init(&bb, &writer);
 	for (i = bb.commits_nr; i > 0; i--) {
 		struct commit *commit = bb.commits[i-1];
@@ -402,7 +431,8 @@ void bitmap_writer_build(struct packing_data *to_pack)
 		struct commit *child;
 		int reused = 0;
 
-		fill_bitmap_commit(ent, commit, &queue);
+		fill_bitmap_commit(ent, commit, &queue, &tree_queue,
+				   old_bitmap, mapping);
 
 		if (ent->selected) {
 			store_selected(ent, commit);
@@ -428,7 +458,9 @@ void bitmap_writer_build(struct packing_data *to_pack)
 		ent->bitmap = NULL;
 	}
 	clear_prio_queue(&queue);
+	clear_prio_queue(&tree_queue);
 	bitmap_builder_clear(&bb);
+	free(mapping);
 
 	trace2_region_leave("pack-bitmap-write", "building_bitmaps_total",
 			    the_repository);